Postgres 13

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DROP TABLE

SELECT 10

igrp	arr[1] ... [9]
9	9,19,...,89
3	3,13,...,83
5	5,15,...,85
4	4,14,...,84
0	10,20,...,90
6	6,16,...,86
2	2,12,...,82
7	7,17,...,87
1	1,11,...,81
8	8,18,...,88

SELECT 10

SELECT
 iGrp
,
 (--subquery
   SELECT DISTINCT _result -- result of the loop
   FROM
   /** while loop AKA tail recurs **/
   (--loop shell
     WITH RECURSIVE _recu AS -- inner loop
       (
         /*top of loop
            recursion top query, acts like variable DECLARE block
          */
         SELECT
           ''::VARCHAR AS _arr2 -- declare accumulator string
          , -1::INT    AS n     -- declare counter var
          , ARRAY_REMOVE( ARRAY[ NULL ]::INT[], NULL )::INT[] AS n_mod9_a -- another var, an array

UNION

/** actual loop work here
              selectively aggregate into new array (_arr2), skipping by mod-9
         **/
         SELECT DISTINCT

CONCAT_WS( ','
                      , _arr2
                      ,     CASE
                            WHEN MOD( n , 9 /** store every 9th position in the array**/ ) = 0
                            THEN a.arr[ n ]::VARCHAR
                            ELSE NULL -- nada
                            END

)  AS  _arr2
          /** increment n by 1 **/
          , n + 1 AS n
          , n_mod9_a  || ARRAY_REMOVE( CASE WHEN MOD( n , 9 ) = 0 THEN ARRAY[n] ELSE NULL::INT[] END, NULL ) AS n_mod9_a

FROM _recu -- inner loop
            , tmpX0 b
          WHERE a.iGrp = b.iGrp -- correlated subquery
           AND n < 100 -- just 100 iter

)-- _recu
        /** get the result from the loop **/
         SELECT CONCAT_WS( '|' , BTRIM( _arr2, ',' ) , '<-loop->' , BTRIM( n_mod9_a::VARCHAR, ',' ) ) AS _result
         FROM _Recu/**inner loop**/
         ORDER BY n DESC -- last result first
         LIMIT 1 -- only the last result
    ) loop_shell
 )--subq
 
FROM tmpX0 a -- "a" of correlated queries, matches to correlated subquery "b" (above)
ORDER BY 1
;

igrp	_result
0	90,180,270,360,450,540,630,720,810,900,990\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
1	81,171,261,351,441,531,621,711,801,891,981\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
2	82,172,262,352,442,532,622,712,802,892,982\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
3	83,173,263,353,443,533,623,713,803,893,983\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
4	84,174,264,354,444,534,624,714,804,894,984\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
5	85,175,265,355,445,535,625,715,805,895,985\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
6	86,176,266,356,446,536,626,716,806,896,986\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
7	87,177,267,357,447,537,627,717,807,897,987\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
8	88,178,268,358,448,538,628,718,808,898,988\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
9	89,179,269,359,449,539,629,719,809,899,989\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}

SELECT 10

igrp	_result
0	90,180,270,360,450,540,630,720,810,900,990\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
1	81,171,261,351,441,531,621,711,801,891,981\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
2	82,172,262,352,442,532,622,712,802,892,982\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
3	83,173,263,353,443,533,623,713,803,893,983\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
4	84,174,264,354,444,534,624,714,804,894,984\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
5	85,175,265,355,445,535,625,715,805,895,985\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
6	86,176,266,356,446,536,626,716,806,896,986\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
7	87,177,267,357,447,537,627,717,807,897,987\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
8	88,178,268,358,448,538,628,718,808,898,988\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}
9	89,179,269,359,449,539,629,719,809,899,989\|<-loop->\|{0,9,18,27,36,45,54,63,72,81,90,99}